In laser processing field, the emergent light diameter Φ of a laser beam is relative small (about 1 mm). If such narrow light beam is focused directly, a greater Rayleigh disk will be obtained. According to the Rayleigh disk formula: δ=2.44λ/D, where δ represents a diameter of the Rayleigh disk, D represents an entrance pupil diameter, f represents a focal length, it can be inferred that, the smaller D, the greater δ, the power on the focus point is weaker, thus greatly reducing a processing accuracy of the system. Therefore, a beam expander is always employed in an optical system employed for a laser processing to expand the narrow light beam emitted from a laser, which will be focused by a laser processing focus lens.
According to the Lagrange invariant theorem: J=nDθ=n′D′θ′, where n and n′ respectively represent refraction indexes of mediums in object field and image field of the optical system, when mediums in the object side and the image side are both air, n=n′=1. D and D′ respectively represent an entrance pupil diameter and an exit pupil diameter of the optical system. θ and θ′ respectively represent field angles of an incident light and an emergent light, which can be represented in radians if they are too small. It can be inferred that, when θ is relative large, the light beam can be expanded by β=D/D′ times via the beam expander for subsequent laser processing.
During the processing, laser wavelengths λ adapted for a variety of processing objects are different, such as λ=1064 nm, λ=532 nm, λ=266 nm, and so on, thus a variety of optical beam expanders adapted for lasers with different wavelengths appear. At the same time, it demands that the optical beam expander has a greater of the application range, i. e. a beam expander can accommodate different application environments, such as different exit pupil diameters, divergence angles of a laser. In addition, the optical beam expander is required to adapt to different laser processing focus lens to achieve an ideal coupling. Therefore, a zoom beam expander becomes a research focus in the field of laser processing. At present, the common magnification of the zoom beam expander is between 2× to 8×, which is too small to satisfy a demand of the laser processing. In the event when a greater zoom value is required, a beam expander with fixed magnification must be used, such that a requirement of multiple zoom values cannot be achieved by merely a single beam expander, which is inconvenience for laser processing and affects the efficiency of laser processing.